cat

Concatenate and print (display) the content of files.

Syntax
cat [Options] [File]...
Concatenate FILE(s), or standard input, to standard output.
-A, --show-all equivalent to -vET
-b, --number-nonblank number nonblank output lines
-e equivalent to -vE
-E, --show-ends display $ at end of each line
-n, --number number all output lines
-s, --squeeze-blank never more than one single blank line
-t equivalent to -vT
-T, --show-tabs display TAB characters as ^I
-u (ignored)
-v, --show-nonprinting use ^ and M- notation, except for LFD and TAB
--help display this help and exit
--version output version information and exit
With no FILE, or when FILE is -, read standard input.

The cat command can be piped into grep to find specific words in the file:cat file.txt | grep keyword output.txt

However all modern versions of grep have this built-in. Running a single command/process is more efficient, and so with large files will be noticably faster:grepkeyword file.txt output.txt

grep can also display an entire file, (like cat), by using the grep keyword "." which will match lines with at least 1 character. Alternatively the grep keyword "^" will match the beginning of every line including blank lines.
When grep is used to display multiple files, it will prepend each line of output with the filename:$ grep . *.txt

Examples:

Display a file:$ cat myfile.txt

Display all .txt files:$ cat *.txt

Concatenate two files:
$ cat File1.txt File2.txt > union.txt

If you need to combine two files but also eliminate duplicates, this can be done with sort unique: $ sort -u File1.txt File2.txt > unique_union.txt

Put the contents of a file into a variable$ my_variable=`cat File3.txt`

"To be nobody but yourself - in a world which is doing
its best, night and day, to make you like everybody else - means to fight the
hardest battle which any human being can fight, and never stop fighting"
~ E.
E. Cummings

Related:

cp - Copy one or more files to another location.mv - Move or rename files or directories.pv - Monitor the progress of data through a pipe.tac - Concatenate and write files in reverse.
hexdump - View binary fileStupid Cat tricks - by Mike Chirico.
Equivalent Windows command: TYPE - Display the contents of a text file.